Hydraulic thrust-transmitting device



F eb. 9, 1965 G. T. RANDOL [mentor formed with an intermediate portion of a tubular workperforming member .PM. The inner cylindrical surface on the power cylinder is defined by a smooth working surface 36 for said piston '70, and an internal shoulder 37 defining a forward unfinished cylindrical surface 38, said power-piston dividing the interior of the power cylinder 10 into a variable pressure power chamber 40 and a variable volume primary supply chamber 41a, said power-piston being biased toward normally released position as shown in FIGURE 1 by a normally preloaded return spring 42.

A filler cap FC is provided for the supply chamber 41a, said cap having an exterior annular flange 43, a reduced diameter externally threaded depending ring flange 44, a coaxial exterior embossment 45 preferably formed with a hexagonal perimeter for reception of a wrench for manual removal and installation of said cap, from an internally complementally threaded opening 45 provided in the wall of the power cylinder into the supply chamber 41a. Centrally depending from flange 43 and concentrically disposed within the iange 441 is another depending ring-like flange 43 internally threaded for reception of an externally threaded hollow plug 49, a coaxially disposed breather passage 51 is provided through the flange 43 and embossment 45 to the exterior, the inner end of said passageway terminating in an annular valve seat 52 of larger diameter, and a coaxial bore 53 through said plug terminates in a conical concavity 55 in which a movable check-valve element such as a steel ball 56, is positioned to cooperate with the valve seat 52 to prevent escape of fluid from the supply chamber 41a should a slightly pressurized condition develop which, if present, would cause the ball to move upwardly and seat tightly thus closing the passageway 51. When the iiuid in the supply chamber 41a is under normal non-activating flow condition, the ball drops under influence of gravity, to vent the supply chamber 41a to atmosphere thus avoiding the tendency of the power-piston PP when operating, to create a vacuum condition within said supply chamber.

Accordingly, if the supply chamber 41a becomes filled and fluid therein slightly pressurized, the check-valve 56 in the liller cap FC becomes effective to restrict uidsurge and therefore prevents tiuid loss via the breather passageway 51. A uid outlet passageway 58 is provided through the end wall 11 for the supply chamber 41a, said outlet passageway extending to the exterior of the power cylinder 10 via an internally threaded boss 59 integral with said end wall which receives a threaded nut vand bushing 60 for attaching a iuid return conduit 61 as shown in FIGURE 1.

The master brake cylinder MC includes the cylindrical casing 90 coaxially disposed medially of the end wall 11 of the power cylinder 10 and preferably integral therewith as shown. The casing 90 has a finished inner cylindrical surface 91 and is open at its inner end and closed at its outer end by a wall 92 through which a disch-arge port 93 is provided. A compensating port 95 is provided through the wall of the casing 90 for maintaining the supply chamber 41a connected to the interior of said casing. A threaded hole 96 through the wall of the power cylinder 10 is fitted with a drain plug 97 which is coaxially positioned with respect to the compensating port 95 to enable processing of this port as by inserting a drill through the hole 96.

A residual pressure check-valve RV having a cupshaped housing 178 with openings 179 provided through its end wall 130, the open end of said housing terminating in an outturned annular ange 181 parallel to the end wall aforesaid, which movably engages a ring-like valve seat 182 engaging the inner side of the end wall 92 of the master cylinder, said check-valve assembly including a self-contained pressure discharge one-way check-valve 183 comprising a flexible disc 1% impinged at its center between cooperating mating portions 105, 106 carried by the partition member 187 which is provided with openings 189, and the end wall 100, respectively. rfhe flexible disc is adapted to control the openings to admit pressurized iiuid from the working chamber 1Z1a into the discharge port 93 Via the openings 189 to operate the wheel cylinders (not shown) of the vehicle wheel brakes (not shown) as is understood.

The tubular work-performing member PM is ported at 99a to maintain the supply chamber 41a connected to 1an intermediate portion of the interior of said work member which is provided with a counterbore 101e terminating at its forward end with an internal annular shoulder 102:1 dening another counterbore 103e merging with the forward axial bore 104m to produce another internal annular shoulder at 105e, said axial bore 104:1 of the tubular work member PM being closed at its forward end oy an end wall 300 to provide a solid piston head on the hydraulic piston g whereby the cup seal 120:1 transmits reaction from the spring 190a via its spring seat 191e to the head end of the work-performing member PM only. Therefore, this spring serves as a piston return spring and control for the residual pressure regulating means since its reaction on piston 145 (see Patent 3,046,746 granted to me luly 3l, 1962) has been nullitied. Slidably mounted in the axial bore 104@ is a hydraulic piston 301 with its rear reduced end V152e piloted in the blind hole 154e in the forward end extension 14ml on a control valve piston 13051, and the forward end of the piston 301 projects into the bore 104@ to provide therewith a hydraulic dashpot generally designated DP having a variable working chamber 302 with respect to the end wall 300. Longitudinally spaced from the forward end of the piston 301 is an annular outstanding flange 147 slidably mounted in said counterbore 103a, said flange cooperating with shoulder 105 to produce a variable volume fluid supply chamber 148 hereinafter referred to as a secondary supply chamber.

The head end of the piston 301 is provided with a peripheral offset at 303 to provide an annular valve seat 304, and a longitudinal counterbore 305 merges with a blind smaller coaxial bore 306 to produce a static valve chamber 300, and a plurality of passageways 307 extends through the Wall of the counterbore 305 to maintain fluid communication between said secondary supply chamber and said valve chamber 30S longitudinally spaced rearwardly of the head end of said piston, and another passageway 309 is provided through the wall of the counterbore 10311 for maintaining the secondary supply chamber aforesaid in communication with the primary supply chamber 41a. A piston ring 310 is provided in an annular groove intermediate the head end and valve chamber 308 of said piston 301 to eifect a fluid-tight seal with respect to said bore 10411 whereby the dashpot chamber 302 receives and discharges the tiuid under control of check-valve means comprising a cup-shaped valve element 311 slidably mounted on a support stem 312 provided with a forward annular flanged end 313, and the rearward normal diameter portion being pressfitted into the bore 305 for movement as a unit with the piston 301, said movable check-valve element 311 having a cylindrical body portion 314 closed at its rear end by a wall 315 centrally apertured at 316 through which said support stem 312 projects and the forward open end of said body portion terminates in an outturned annular ange 317 in parallel relationship to the end wall thereof to overlie the aforesaid annular valve seat 304 and cooperate therewith under influence of a normally preloaded light compression spring 318 operably disposed within the body portion in encircling relation with respect to the forward portion of the support stem, between the flange 313 and end wall 315. The wall of the body portion 314 is provided with a metering control port 319. A cross pin 321 through the piloted end of said piston 301 couples the latter to the control valve piston e for axially movement as a unit.

3,1es,sia

In operation, initial operator pressure applied to another actuatable member disclosed herein as the conventional pivotally suspended pedal P is transmitted to the control valve piston 130e via a push-rod PR. Resistance provided by the trapped liuid in the dashpot chamber 392 due to restriction to outflow by the metering port 319 back into the supply chamber Alla via the valve chamber 308, passageways Sil?, Sill?, prevents premature relative movement of the control valve piston llla with respect to the hydraulic working member PM to induce operation of the booster motor HM, such resistance being supplemented by the preloaded reaction from the valve return spring lilla. In this way, initial pedal depression is effective particularly when rapidly executed to move the work-performing member PM substantially as a unit therewith relatively to the power-piston assembly PP to take up the slack inthe brake system thus placing the brake shoes in light contact with their respective Wheel drums 'and thereby subject said work element to a workload resistance opposing pedal-actuation thereof. With increasing pressure applied to the pedal (not shown) at this point of the braking operation, the dashpot D? continues to cooperate with the spring lila to pressurize the Huid in the working chamber 12in of the master cylinder MC therefore the associated brake system sufficiently to substantially halt movement of the work member PM thus inducing relative pedal movement with consequent yielding of the spring llla without interrupting a progressively diminishing resistance from the pressurized condition of the fluid in the dashpot chamber 3h25. At this stage of initial braking application, the operator is apprised of the inauguration of the power phase since the dashpot DP and spring llla are reacting to oppose pedal movement in such manner that if pedal movement is moderately rated, themergence of the power phase with such initial operator movement is smoothly accomplished, but if the operator imparts a rather rapid or erratic initial movement to the pedal, the dashpot comes into play to momentarily block said relative movement of the control valve piston liila with respect to said hydraulic working member PM irrespective of the preloaded weight of the valve return spring ltla, to prevent a sudden initial energization of the hydraulic servomotor HM with consequent grabbing or locking of the wheel brakes (not shown). Under these circumstances, the operator merel' ly converts pedal movement into pressurizing the luid in the working chamber 12in and therefore, effects initial brake application in proportion to the speed of pedal movement until the dashpot chamber db2 can discharge its trapped lluid via the metering port Sill, valve chamber 'v 398, and passageways 3637, 3h@ back into the supply chamber 41a; but, if the operator gives the pedal a more slowed normal rate of initial movement, the metering port 3l9 enables the trapped liquid in the dashpot chamber S to discharge substantially in proportion to pedal movement displacement thereof so that unitary movement of the control valve element 1:50a with the pedal will effect interruption of the fluid circulating system to energize the hydraulic power-piston 70 in a gradually increasing rate of movement to smoothly blend the power phase with initial operator phase thereby producing a smooth braking application through the full range of pedal movement.

By virtue of the blocking function provided by the dashpot DP, fortuitous energization of the hydraulic servomotor HM is avoided, and a light valve return spring llla may therefore be used to render the present brakebooster apparatus A highly suitable for low-pedal control so that the operator may use a swinging movement of the toot from the heel engaging the lioor in the drivers compartment, between the accelerator and brake pedals substantially in the same plane when in normal released positions. By using the toe portion of the foot, improved control action is provided with reduced eliort, and

safety of operation of the vehicle is enhanced as well by A some reduction in the time required to apply pressure on the brake-pedal.

When pressure is removed from the pedal to take the brakes oth valve piston return spring 1505: alone biases the control valve piston ltla toward its normally released position as shown in FIGURE 1, and this releasing operation is rapid to eect quick release of the brakes since the dashpot chamber 362 is relilled via the valve flange 317 unseated from its seat 3M which enables a much larger volume of tluid to re-enter the chamber 392 than that capable of being discharged through the metering port Slg during a brake-applying movement of the dashpot piston Elli. A vacuum condition temporarily created in the dashpot chamber 392 unseats the valve element Sil against the force of its seating spring 318 to draw the from the supply chamber la via the secondary Supply and valve chambers 148, 393 respectively, in such volume so as to progressively relill the dashpot chamber lf2-@3 as it enlarges to normal size shown in FIGURES l and 2 without restricting pedal movement as foot pressure is removed from the pedal therefore the control valve 13h61 and dashpot piston Sill. Therefore, the valve return spring *i5-Sn ellccts withdrawal of the control valve piston ilcz to its normal open position as shown in FIGURE l, the spring being oi such strength so as to move the alve element in unison with the release of the pedal, and the open condition or the check-valve Sill enables ingress of fluid from the supply chamber fila into the dashpot chamber 392 to bypass the metering port 3l@ so that the suction of the piston Sill induced by the valve return spring ldlla progressively relills chamber 362 as rapidly as the pedal is released to take the brakes Lioff'!! t" Accordingly, it is seen from the foregoing description of this hydraulic dashpot device, that the dashpot arangenient serves a twofold purpose of transmitting operator force to initially apply [the brakes if the speed of pedal movement is excessive, and a lighter valve return spring may be combined with the dashpot action to provide a lighter pedal operation favoring a low-pedal mechanismn in which less mechanical advantage is provided over a normal height pedal.

Thus when initial operator force is applied as, for eX- aniple, to the brake-pedal, P, the piston 3*-@1 is moved substantially in unison with the control valve piston lilla, whereupon reduction in the size of the dashpot chamber 3612 ensues as a result of fluid displacement therefrom through the metering port 319 into said secondary chamber 3h55 and thence via the passageways 3137, bach into the primary supply chamber dla. This slowed reduction in the size of said dashpot chamber 392 produces a progressively diminishing resistance to movement of the brake-pedal and connected control valve piston l3nt! relative to the work member PM, to

induce operative energization of the booster motor HM and at the same time provide the operator with physical awareness of the amount of working pressure in effect for a given position of` the brake-pedal. Upon release of the brake-pedal, the piston Sill retracts accordingly under inuence of its return spring lltla, which enables the check-valve to move against its spring load to unseat, thereby enabling the dashpot chamber 302 to fill rapidly with little or no resistance to return pedal movement. During this normalizing of the size of the dashpot chamber a partial vacuum condition may obtain which augments the inflow of iiuid from the valve chamber 39S to said dashpot chamber 302 so that retraction of the piston Still is unimpeded during releasing movement of the brake-pedal, but when the pedal is depressed to apply the vehicle brakes, movement of the piston Sill is gradual due to seat condition ol' the check-valve Sill wherein transfer to tluid from the dashpot chamber t0 valve chamber 3tlg is by way of themetering port" Pumping of the brake-pedal to preventing fade due to excessive heating of the brake drums, is especially aided by my novel hydraulic dashpot device DP since the brakepedal can be pulsated by the foot of the operator such that releasing movement is substantially unresisted, while depressing movement to apply the brakes is opposed at any station of the pedal through its full operating stroke to provide predictable control over the total braking effort without such opposition, the operator would invariably apply too much pressure to the pedal thus rendering the brake applications over-sensitive therefore diflicult to control.

In view of the foregoing description augmented by the drawing, it should be evident that my invention discloses a novel hydraulic thrust-transmitting device having special use in pressure differential operated servomotors adapted to provide the major portion of the operating force to actuate the vehicle brakes, and that the structure is simple in design and arrangement, and which is adapted to successfully achieve its objectives.

Whereas, the drawing illustrates, and the description sets forth the preferred embodiment of my invention, it should be understood that the invention contemplates any Variations of structure, and equivalents or modifications which may fall within the scope of the subjoined claims.

Having thus described my invention, I claim:

1. In a hydraulic thrust-transmitting device having a fluid supply chamber, and adapted for use in cooperation with a hollow member activatable to perform work in a thrust-transmitting direction and which is closed at one end and open at the other, said member being activatable under influence of a pair of cooperable actuatable members, with one of said pair of members being adapted to control the other member, the improvement which comprises: a piston reciprocable in the hollow of said work member under influence of said one of said pair of members; a variable volume hydraulic dashpot chamber provided between the closed end of said work member and confronting end of said piston; a valve chamber incorporated in the confronting end of said piston, and provided with an open end communicating with said dashpot chamber, and a closed end; fluid passageway means between said supply and valve chambers; valve means including an element movably mounted on the confronting end of said piston and biased thereagainst under springaction to control fluid flow between said dashpot and valve chambers aforesaid; a valve seat defining the confronting end of said piston for said valve element to seat against; and a fluid flow metering port through a portion of said valve element interconnecting said dashpot and valve chambers, said port functioning when said Valve element is seated during thrust-transmitting activation of said work member, to restrict excursion of fluid from said dashpot chamber into said valve chamber, and thereby enabling substantially unitary movemnet of said piston and work member under initial actuation of said one actuatable member until said work member encounters a resistance to movement of suicient magnitude to induce relative actuation of said one actuatable member with respect to said work member to control actuation of the other acutatable member whereupon joint actuation of both actuatable members is effective to activate said work member to transmit additional thrust against said resistance.

2. In a hydraulic thrust-transmitting device having a fluid supply chamber, and adapted for use in cooperation with an element movable to perform work, said element being activatable in a thrust-transmitting direction under influence of a pair of cooperable actuatable members, with one of said pair of members being adapted to control the other member, the improvement which comprises: a control element movable initially in unison with said one member and said work element and subsequently relatively to the latter element to control actuation of said other member; fluid thrust-transmitting means including a check-valve therefor movable between restric. tive and free flow positions of control and disposed in coaxial relationship with respect to said control element between said latter element and said work element, said thrust-transmitting means functioning in a thrust-transmitting direction to releasably resist relative actuation aforesaid of said one member and said control element as a unit with respect to said work element when said check-valve is in restrictive position during initial actuation of said one member until said work element encounters a predetermined work resistance to movement requiring addiltional actuation of said one member to overcome said predetermined resistance to enable relative movement aforesaid of said control eiement; passageway means including interposition of said check-valve to control the same, between said supply chamber and thrust-transmitting means; and a normally preloaded spring operably incorporated between said control element and said check-valve to bias the latter toward restritcive position wherein excursion of fluid flow from said thrust-transmitting means to said supply chamber is releasably blocked.

3. ln a hydraulic thrust-transmitting device having a fluid supply chamber, and adapted for use in cooperation with an element movable to perform work, said element activatable in a thrust transmitting direction under in- ;dnence of a pair of cooperable actuatable members, with one of said pair of members being adapted to control the other member, the improvement which comprises: a control element movable initially in unison with said one member and said work element and subsequently relatively to the latter element to control actuation of the other member; fluid thrust-transmitting means including a check-valve therefor movable between restrictive and free flow positions of control and disposed in coaxial relationship with respect to said control element between said latter element and said work element, said thrusttransmitting means functioning in a thrust-transmitting direction to releasably resist relative actuation aforesaid of said one member and said control element as a unit with respect to said work element when said check-valve is in restrictive position during initial actuation of said one member until said workelement encounters a predetermined work resistance to movement capable of inducing relative movement aforesaid of said control element; passageway means including the interposition of said check-valve to control the same, between said supply chamber and thrust-transmitting means; a normally preloaded spring operably incorporated between said control element and said check-valve to bias the latter toward restrictive position wherein excursion of fluid flow from said thrust-transmitting means to said supply chamber is releasably blocked; a fixed abutment; and another normaliy preloaded spring reacting between said abutment and work element to transmit a progressively increasing reactive force to supplement said work resistance.

4. In a hydraulic thrust-transmitting device for use in cooperation with a tubular work-performing element closed at one end and open at the other, said element being activatable in a thrust-transmitting direction by a pair of cooperable actuatable members, with one of said pair of members being adapted to control the other member, the improvement which comprises: a primary fluid supply chamber; a piston reciprocable in said work element; a variable volume hydraulic dashpot chamber rovided between the closed end of said work element and the end of said piston projecting therein; a cylindrically walled valve chamber formed in said piston in coaxial relationship to said dashpot chamber, with one end closed and the open end confronting said dashpot chamber; an annular valve seat terminating the wall of the open end of said valve chamber; check-valve means including a movable element adapted to cooperate with said valve seat under spring action to isolate said valve chamber from said dashpot chamber; a variable volume secondary fluid supply chamber incorporated between said piston and work element in concentric overlapping relationship with respect to said piston; a port in said work element for maintaining fluid communication between said two supply chambers; a port through the wall of said valve chamber for maintaining fluid communication between said valve and secondary supply chambers; a normally preloaded spring reacting between said work element and said piston to establish the latter in normal position wherein said dashpot chamber is normalized and said check-valve element seated under influence of its spring action; means interconnecting said one actuatable member with said piston for movement as a unit in opposition to the bias of said preloaded spring to reduce the size of said dashpot chamber; and a restrictive port in the check-valve element for retarding the rate of fluid flow from the dashpot chamber into said valve chamber to thus produce a dashpot-aetion for releasably resisting relative movement of said piston with respect to said work element under intluence of said one actuatable member, whereby a portion of the initial force transmited by said latter member is applied to activate said work element substantially in unison therewith until the latter element encounters a predetermined work resistance to movement prior to the other actuatable member becoming operative under control of the one actuatable member actuating relatively to said work element as a function of said work resistance to enable joint activation of said work element by both actuatable members.

5. A hydraulic thrust-transmitting device constructed in accordance with claim 4 in which said check-valve means additionally comprises said movable element formed as a cup-shaped member having a forward outstanding annular ange and a longitudinally spaced rear end wall interconnected by a cylindrical sleeve, said ilange normally engaging the aforesaid annular valve seat; a central aperture in the end wall deiined by a reduced diameter portion of said sleeve formed by diminishing the normal diameter of the latter; a support rod xed on said piston and projecting coaxially through said aperture and secondary supply chamber to slidably support said cup-shaped member; an annular head terminating the forward end of said support rod; and a normally preloaded compression spring operably disposed within said cylindrical sleeve in encircling relation to that portion of the support rod normally Iin circular alignment therewith, to react between said rod head and end wall to produce said spring-action to effect seating of said flange on said annular valve seat thereby rendering said dashpot chamber effective to releasably oppose movement of said piston relative to said work element under inuence of said one actuatable member.

6. ln a hydraulic thrust-transmitting device adapted for use in cooperation with a tubular work-performing member closed at one end and open at the other, said member being activatable in a thrust-transmitting direction under joint influence of a pair of cooperable actuatable members, with one of said pair of members being adapted to control the other member, the improvement which comprises: primary and secondary concentrically overlapping fluid supply chambers; a blind axial bore in said work member which opens into a counterbore delining said secondary supply chamber; a reactive piston having a normal position and provided with an outstanding annular frange at its rear end and which sli-dably projects into said counterbore; an internal annular shoulder' defined by the juncture of said blind bore and counterbore; a normally preloaded compression spring encircling that portion of the body portion of said piston coextensive with said secondary supply chamber; a variable dashpot chamber produced by the said piston projecting into said blind bore in normally spaced relation to the blind end thereof; another blind axial bore in said piston defining a cylindrically walled valve chamber closed at its rear end and operi at its forward end confronting said dashpot chamber; a valve seat deiining the open end of the walled valve chamber; a springloaded chec -valve normally effective to close the open end of said valve chamber by engagement with said valve seat to isolate the said dashpot chamber from said valve chamber; a port through a porti-on of the said checkvalve for maintaining restrictive fluid communication between said dashpot and valve chambers aforesaid when said check-Valve flange is seated whereby initial actuation of said one actuatable member to activate said Work member as a unit acts on the uid in said dashpot chamber to reduce the size thereof thus forcing iluid ,therefrom through said restrictive port in the check-valve into said valve chamber to provide releasable resistance to movement of said piston relative to said work member and to simultaneously apply thrust thereto; a port through the Work member for maintaining iluid communication between said secondary and primary supply chambers; and another normally preloaded spring positioned in said secondary supply chamber to react between said annular flange and shoulder to bias said piston toward normal position upon inactivation of said Work member in response to release of said one actuatable member, whereby said check-valve moves against its spring load to connect said valve chamber with the dashpot chamber for rapid refilling of the latter chamber as a function of a partial vacuum obtaining in said dashpot chamber produced during retraction of said piston relative to said work member.

References Cited in the tile of this patent UNITED STATES `PATENTS 1,887,287 Churchill Nov. 8, 1932 2,112,952 Casper Apr. 5, 1938 2,183,003 Becker Dec. 12, 1939 FORElGN PATENTS 1,119,575 France Apr. 3, 1956 

1. IN A HYDRAULIC THRUST-TRANSMITTING DEVICE HAVING A FLUID SUPPLY CHAMBER, AND ADAPTED FOR USE IN COOPERATION WITH A HOLLOW MEMBER ACTIVATABLE TO PERFORM WORK IN A THRUST-TRANSMITTING DIRECTION AND WHICH IS CLOSED AT ONE END AND OPEN AT THE OTHER, SAID MEMBER BEING ACTIVATABLE UNDER INFLUENCE OF A PAIR OF COOPERABLE ACTUATABLE MEMBERS, WITH ONE OF SAID PAIR OF MEMBERS BEING ADAPTED TO CONTROL THE OTHER MEMBER, THE IMPROVEMENT WHICH COMPRISES: A PISTON RECIPROCABLE IN THE HOLLOW OF SAID WORK MEMBER UNDER INFLUENCE OF SAID ONE OF SAID PAIR OF MEMBERS; A VARIABLE VOLUME HYDRAULIC DASHPOT CHAMBER PROVIDED BETWEEN THE CLOSED END OF SAID WORK MEMBER AN CONFRONTING END OF SAID PISTON; A VALVE CHAMBER INCORPORATED IN THE CONFRONTING END OF SAID PISTON, AND PROVIDED WITH AN OPEN END COMMUNICATING WITH SAID DASHPOT CHAMBER, AND A CLOSED END; FLUID PASSAGEWAY MEANS BETWEEN SAID SUPPLY AND VALVE CHAMBERS; VALVE MEANS INCLUDING AN ELEMENT MOVABLY MOUNTED ON THE CONFRONTING END OF SAID PISTON AND BIASED THEREAGAINST UNDER SPRINGACTION TO CONTROL FLUID FLOW BETWEEN SAID DASHPOT AND VALVE CHAMBERS AFORESAID; A VALVE SEAT DEFINING THE CONFRONTING END OF SAID PISTON FOR SAID VALVE ELEMENT TO SEAT AGAINST; AND A FLUID FLOW METERING PORT THROUGH A PORTION OF SAID VALVE ELEMENT INTERCONNECTING SAID DASHPOT AND VALVE CHAMBERS, SAID PORT FUNCTIONING WHEN SAID VALVE ELEMENT IS SEATED DURING THRUST-TRANSMITTING ACTIVATION OF SAID WORK MEMBER, TO RESTRICT EXCURSION OF FLUID FROM SAID DASHPOT CHAMBER INTO SAID VALVE CHAMBER, AND THEREBY ENABLING SUBSTANTIALLY UNITARY MOVEMENT OF SAID PISTON AND WORK MEMBER UNDER INITIAL ACTUATION OF SAID ONE ACTUATABLE MEMBER UNTIL SAID WORK MEMBER ENCOUNTERS A RESISTANCE TO MOVEMENT OF SUFFICIENT MAGNITUDE TO INDUCE RELATIVE ACTUATION OF SAID ONE ACTUATABLE MEMBER WITH RESPECT TO SAID WORK MEMBER TO CONTROL ACTUATION OF THE OTHER ACTUATABLE MEMBER WHEREUPON JOINT ACTUATION OF BOTH ACTUATABLE MEMBERS IS EFFECTIVE TO ACTIVATE SAID WORK MEMBER TO TRANSMIT ADDITIONAL THRUST AGAINST SAID RESISTANCE. 